1. Field of the Invention
The invention relates in general to image processing. More particularly, this invention relates to a method and apparatus that accelerates color space conversion by employing a multi-dimensional and multi-port memory.
2. Description of the Related Art
Color Copiers or other imaging devices capture an image from an input device and attempt to create a suitable duplicate on an output device.
The input device has inherent properties and characteristics with regard to the sensing process in a document. An example of an input device is a Charged Coupled Device (“CCD”) scanner. A CCD sensor is typically composed of three sensors for each pixel to be sensed. Each of the three sensors responds to a different wavelength of light. Typically, the wavelengths are close to Red, Green and Blue (RGB).
The output device has inherent characteristics with regard to the process of creating an image. One such example is a color laser printer. The output device typically creates an image using four different colors, i.e. Cyan, Magenta, Yellow and Black (CMYK).
Color Management is defined as the task of accurately converting color information of one form or space to that of another space. This might be compared to the way that points in space or in a graph can be converted from a Euclidean space to a Spherical or other coordinate system.
The classical “textbook” method for converting color from one representation to another is by matrix multiplication. A three-component (e.g. RGB) element is multiplied by 3×3 matrix to generate a value in the new space. Industry standards exist for these matrices, such as those used in color television or JPEG compression.
The matrix-multiplication method demands that the conversion process be rather ordinary, in that the three-dimensional surface generated is continuous and without irregularities. For best results, one might desire that a different 3×3 matrix value set be used to convert various regions of the input color space. This is rather cumbersome, and the typical implementation is by table lookup.
The lookup process uses a table, usually implemented in memory such as SRAM or DRAM, that provides an output value for every input value. However, such a method requires a large amount of memory. For example, table-based color space conversion from RGB to CMYK would ideally translate every incoming 24-bit RGB value to a unique CMYK value. This would require a table with 16 million entries of 32 bits, or 64 MB of memory.
Due to practical limits on the size of the memory that can be used to store the color lookup table, only discrete data points, or nodes, are stored in the color lookup table. As a result, it is often necessary to interpolate between a set nodes adjacent to a given image color to determine the desired output color for a destination device.
Typically, the adjacent nodes are separately accessed from the lookup table, thus requiring multiple accesses to memory. For example, in the three-dimensional RGB input space, a set of eight nodes forming a cube around a color pixel must be accessed. This proves to be inefficient, given the plurality of pixels in input images.
Therefore, the need arises for an image processing device that allows for a single access of memory for interpolation of data points, thereby expediting input image conversion.
In U.S. Pat. No. 6,246,396, Gibson discloses an apparatus for converting an input image in an input color space to an output image in an output color space. Gibson does not interleave memory by placing odd and even indices into separate lookup tables. The subject invention separates odd and even indices for all three dimensions, resulting in eight separate lookup tables.